Method For Coating Light Alloy Rims

ABSTRACT

The present invention relates to a method for coating light alloy rims, to coating materials for use in this method, and to the coated light alloy rims obtained in this way.

The present invention relates to a method for coating light alloy rims,to coating materials for use in this method, and to the coated lightalloy rims obtained in this way.

For coating, light alloy rims are typically primer-coated withpowdercoating materials, which are applied in powder form, dried, meltedat an elevated temperature to form a film, and subsequently cured.

One disadvantage associated with this coating procedure is that dryingand melting necessarily involve a large amount of time, and henceconveyor length in a continuous process, and this reduces the cycle timein coating, and increases the conveyor length in a continuous process.

Another disadvantage is that drying, melting and baking of apowdercoating layer, with a thickness generally of just a fraction of amillimeter, in the oven involves the heating of the entire rimstructure. For this operation, therefore, substantially more energy isexpended than is necessary for the actual powdercoating layer.Furthermore, the high thermal load impacts adversely on the mechanicalstability (metal microstructure), in a way which may be manifested inthe development of pores or even hairline cracks in the rim structure,producing rim structures which have to be removed as rejects.

Atop the powdercoating primer, further layers (typically basecoat andclearcoat) of solventborne coating material are applied, with an organicsolvent. These layers give off this solvent on drying, resulting inemissions of volatile organic compounds (VOC) and necessitating numerousworkplace safety and environmental measures.

The object of the present invention, therefore, was to provide a methodfor coating light alloy rims that allows the energy and/or time consumedin drying, film formation and/or curing to be reduced and/or the use ofvolatile organic compounds to be reduced.

The object has been achieved by means of a method for coating lightalloy rims, in which a machined light alloy rim blank comprising a castaluminum alloy is coated with at least three layers, comprising primer(A), base coat (B), and clear coat (C), each of which are differentcoating materials, the primer (A) comprising at least oneradiation-curable coating material having an acid number in accordancewith DIN EN ISO 3682 (by potentiometry) of 10 to 120 mg KOH/g, saidcoating material being applied to the substrate directly, and aradiation-curable clear coat layer having a double-bond density offree-radically polymerizable reactive groups per unit mass of coatingmaterial of at least 1 mol/kg being applied to the outside of the lightalloy rims, and said coating material is cured by radiation.

Curing the coating material by means of radiation, in accordance withthe invention, allows a significant reduction to be made in the timeneeded for curing, as compared with baking. Furthermore, it issufficient to cure only the coating material by radiation, therebydispensing with the energy requirement for the heating of the entire rimstructure in the conventional process, and also with the thermal stress.

The light alloy substrates are generally cast aluminum alloys which aswell as aluminum as the principal component include secondary amounts ofsilicon, minor amounts (generally well below 1% by weight) of, forexample, magnesium, titanium and/or iron, and no more than traces(generally well below 0.1% by weight) of other components, examplesbeing strontium, manganese, zinc, tin, copper, nickel, boron, calcium,sodium and/or phosphorus.

The alloys involved are preferably cast aluminum alloys with a siliconcontent of not more than 15% by weight, more preferably of 5% to 15% andvery preferably of 5% to 13% by weight. In certain cases, wroughtaluminum alloys with a silicon content of below 5% by weight areconceivable, although less preferred.

Also preferred are cast aluminum alloys with a copper content of lessthan 0.1%, more preferably less than 0.05%, and very preferably lessthan 0.03% by weight.

After casting, the blanks are generally cleaned, any cast edges andflash is removed, and they are prepared mechanically for their seating;i.e., holes are drilled for the wheel bolts (bolt-hole circle bores) andaxle hub (bearing surface), the rim bed is turned, and the rims arecentered.

Subsequently, in general, the surface is provided with a conversioncoat: preferably, the surface is subjected to zinc phosphating orzirconium dioxide deposition, or is treated with chromium(VI) complexes,chromium(III) complexes or titanium complexes. Also conceivable areconversion coats which are free from toxic heavy metals, comprising, forexample, organic silane polymers, as described in DE 10 2009 001372 A1,for example.

The machined blanks coated in this way, for example, can be introducedinto the method according to the invention. In this case, preferably,the bolt-hole circle bores are at least partly masked prior to coating,so that the areas on the rims against which the wheel bolts bear remainuncoated.

In one preferred embodiment the machined blanks are coated with at leastthree layers, each of different coating materials.

In the case of three layers, the functions these layers take on arethose of primer (A), basecoat (B), and clearcoat (C), and they areapplied in this order, it being possible for each of these three layersto be applied one or more times and for the coating materials in eachlayer to be the same or different. Between the individual applicationsit is possible for the layers to be partly or fully cured, but it mayalso be sufficient in each case merely to carry out drying andoptionally no more than partial curing, and to carry out a complete cureonly at the end. Also conceivable is the application of a layer ofcoating material to an undried and uncured underlying layer.

In one preferred embodiment of the present invention, at least theouter-lying clearcoat layer comprises a radiation-curable coatingmaterial having free-radically polymerizable reactive groups.

In another preferred embodiment of the present invention, at least theprimer comprises at least partly, preferably completely, aradiation-curable coating material having free-radically polymerizablereactive groups.

In one particularly preferred embodiment of the present invention, thelight alloy rim is coated with at least two different radiation-curablecoating materials having free-radically polymerizable reactive groups,and very preferably in this case the primer and clearcoat layer at leastpartly, preferably completely, comprise radiation-curable coatingmaterials.

In a further particularly preferred embodiment of the present invention,the primer and the basecoat and clearcoat comprise radiation-curablecoating materials.

Primer (A)

The primer layer covers preferably the entire rim apart from the bearingsurface and the bolt-circle holes, with the aim of providing protectionfrom corrosion and/or from stonechipping.

The primer (A) generally has a dry layer thickness of 100 to 200 μm,preferably 120 to 180 μm, and is constructed in turn of at least onelayer, preferably of one to four, more preferably of two or three, andvery preferably of three layers (A1) to (A3), each of these layershaving a thickness of 20 to 100 μm, preferably of 50 to 70 μm.

As an example, a primer (A) having two to three layers (A1), (A2) and(A3) is described:

The binders in the coating materials for these layers are, in onepreferred embodiment, binders which are fully radiation-curable andcontain no substantial fractions of organic solvents.

The coating material for layer (A1) is preferably a radiation-curablecoating material comprising

-   (A1a) at least one radiation-curable binder, preferably selected    from the group consisting of aliphatic urethane (meth)acrylates,    aromatic urethane (meth)acrylates, and epoxy (meth)acrylates,-   (A1b) at least one reactive diluent, and-   (A1c) preferably at least one anticorrosion pigment and/or corrosion    inhibitor.

In one preferred embodiment the coating material for the layer (A1) hasan acid number in accordance with DIN EN ISO 3682 (by potentiometry) ofat least 20 mg KOH/g, particularly preferably of at least 40 mg KOH/g.The acid number is preferably not more than 100 mg KOH/g, particularlypreferably not more than 80 mg KOH/g and very particularly preferablynot more than 70 mg KOH/g.

This preferred acid number allows the adhesion of the layer (A1) to thelight alloy surface to be improved. It is unimportant here whether theacid number derives from component (A1a) and/or (A1b). Preferred acidgroups here are carboxyl groups, phosphoric acid groups and phosphonicacid groups, preferably phosphoric acid groups or carboxyl groups.

In another preferred embodiment, the coating material for the layer (A1)has a viscosity at application temperature in accordance with DIN EN ISO3219 (cone/plate system, shear rate 100 s⁻¹) of 60 to 1000 mPas,preferably of 100 to 300 mPas.

The function of the layer (A1) is to promote adhesion of the coatings onthe light alloy substrate, to ensure protection from corrosion, and, bymeans of elasticity, to ensure protection against stonechipping.

The coating material for the layer (A2) may be composed as that for thelayer (A1), but it is possible with preference to forego a particularacid number, since adhesion of the layer (A2) directly on the lightalloy surface is no longer necessary, all that is necessary insteadbeing interlayer adhesion between the layers (A1) and (A3) (if a 3rdprimer layer is applied).

Accordingly, the coating material for the layer (A2) is preferably aradiation-curable coating material comprising

-   (A2a) at least one binder selected from the group consisting of    aliphatic urethane (meth)acrylates, aromatic urethane    (meth)acrylates, and epoxy (meth)acrylates,-   (A2b) at least one reactive diluent, and-   (A2c) preferably at least one anticorrosion pigment and/or corrosion    inhibitor.

Since the layers of the primer (A) in one preferred embodiment areapplied to the entire surface of the rim, in other words both to thefacing side and to the inside, but the layers (B) and (C) are appliedonly to the facing side, the primer represents the sole coating on theinside of the rim.

Consequently, for the inside of the rims, the layer (A2) or (A3) of theprimer (A) represents the sole protective layer against the influenceof, for example, oil, gasoline, brake fluid or rim cleaner, and ought tohave a corresponding chemical resistance, as typically specified in theVDA [German Automakers Association] Guidelines 621-415.

The coating material for the layer (A3) may preferably have acomposition as follows:

-   (A3a) at least one binder selected from the group consisting of    epoxy, polyester or urethane (meth)acrylates and-   (A3b) at least one reactive diluent.

In one preferred embodiment, some or all of these layers may bepartially cured following application, so that the subsequent layer isapplied to the underlying layer which has not been completely cured andtherefore, optionally, is slightly tacky. This has the advantage ofimproving the adhesion between the individual layers.

Basecoat (B)

In the basecoat layer (B), as the coloring layer, there is at least onepigment and/or metallic flakes, responsible for the external appearance.

The coating material for the basecoat may, in one embodiment of thepresent invention, comprise a nonradiation-curable, solvent- orwater-based, preferably water-based, one-component or two-componentcoating material, preferably a two-component coating material.

In another embodiment of the present invention it may be a water-based,radiation-curable coating material.

Preferably it is a water-based two-component coating material.

Relevant feature of the coating material is a solids content of between20% and 80% by weight, preferably 25% to 60%, and more preferably 30% to50%, by weight, based on the nonvolatile fractions of binder, pigment,and, optionally, additives.

In one preferred embodiment of the present invention, the coatingmaterial in question is substantially dried after application, with noabsolute need for curing, particularly for complete curing, though thiscan be carried out.

Preferred water-based two-component polyurethane coating materials arethose coating materials comprising at least one polyisocyanate, as isdescribed in, for example, WO 2011/061314, page 5 line 29 to page 11line 21, and at least one polyol component, as is described in, forexample, WO 2011/061314, page 12 line 14 to page 17 line 31, each herebyincorporated by reference to become part of the present disclosurecontent. The water-emulsifiability of the polyisocyanate component canbe achieved by addition of external emulsifiers, these being emulsifierswhich substantially do not react with the polyisocyanate component, orusing built-in emulsifiers, these being compounds which carry not onlyhydrophilic nonionic, anionic or cationic groups but also at least oneisocyanate-reactive group. Nonionic groups are frequently polyalkyleneoxide ethers, preferably polyethylene oxide ethers; anionic groups maybe, for example, carboxylate, sulfonate, phosphonate or phosphategroups; and cationic groups may be ammonium groups, for example.

Preferred one-component coating materials are cellulose acetobutyrates,examples being those having on average 0.5 to 2.1 acetyl groups and 2.3to 0.6 butyryl groups per glucose unit, and also polyurethanedispersions.

Conceivable, though less preferred, are solvent-based two-componentpolyurethane coating materials as well, comprising at least onepolyisocyanate, as described in, for example, WO 2011/061314, page 5line 29 to page 11 line 21, and at least one polyol component, asdescribed in, for example, WO 2011/061314, page 12 line 14 to page 17line 31, each hereby incorporated by reference to become part of thepresent disclosure content.

Conceivably, albeit less preferably, the layer (B) may be composed of awater-based, radiation-curable coating material comprising

-   (B1) at least one water-emulsifiable, radiation-curable binder,    preferably selected from the group consisting of aliphatic urethane    (meth)acrylates and polyester (meth)acrylates,-   (B2) at least one reactive diluent,-   (B3) at least one effect pigment and/or other pigment, and-   (B4) water,    the fraction of the compounds (B1) to (B3) as a proportion of the    total amount of the compounds (B1) to (B3) being 20% to 80% by    weight.

The water-emulsifiability of the binders can be achieved by addition ofexternal emulsifiers, these being emulsifiers which are substantiallynot incorporated into the binder, or, preferably, using built-inemulsifiers, these being compounds which are built chemically into thebinder and carry at least one hydrophilic nonionic, anionic or cationicgroup.

Nonionic groups are frequently polyalkylene oxide ethers, preferablypolyethylene oxide ethers; anionic groups may be, for example,carboxylate, sulfonate, phosphonate or phosphate groups; and cationicgroups may be ammonium groups, for example.

The coating materials described may be applied one or more times to theexisting layer (A), and ought to be dried after each application, sothat the solvent is substantially removed from the applied layer.Typical drying conditions are, for example, 5 to 60 minutes, preferably5 to 45, more preferably 10 to 30, and more particularly 10 to 20minutes, at a temperature of 40 to 120, preferably 50 to 100, and morepreferably 60 to 80° C.

It is not absolutely necessary here to heat the entire rim to thistemperature; it may also be sufficient to carry out drying in a streamof hot air or using IR or NIR sources, with NIR radiation here beingelectromagnetic radiation in the wavelength range from 760 nm to 2.5 μm,preferably from 900 to 1500 nm.

Also conceivable is low-temperature drying, having the potentialadvantage that solvent removed could be at least partly condensed,contributing to a reduction in the emissions of volatile organiccompounds (VOC).

It is important that at least 75% by weight of the solvent contained inthe coating material is removed by the drying operation, preferably atleast 80%, more preferably at least 85%, very preferably at least 90%,and more particularly at least 95% by weight.

If drying is accompanied already by at least partial curing of thecoating material, this is not a disadvantage, but instead is entirelydesired. It is, however, not absolutely necessary to carry out fullcuring of the applied coating materials.

In one specific embodiment of the present invention, the coated rim canbe mechanically aftertreated again after drying and curing of thecoating material (B), but before application of the clearcoat layer (C),by eroding the coating on the outside (facing side) of the rims by meansof turning, grinding and/or polishing, for example, and exposing themetal. This lustrous metal layer contrasts with the pigmented coating(B) and can then be coated in the next step directly with the clearcoat(C).

Clearcoat (C)

The clearcoat layer (C) is preferably applied only on the outside/facingside of the rims.

The function of the clearcoat is a combination of the properties ofweathering resistance, chemical resistance, and scratch resistance.

In one preferred embodiment the coating material for this layer as wellis entirely a radiation-curable coating material having free-radicallypolymerizable reactive groups. The coating material may optionallycontain up to 10% by weight of at least one solvent if the latter isrequired as a flow control assistant or as a viscosity-loweringcomponent.

The function of this clearcoat layer is to produce weathering andchemical resistance and also scratch resistance.

In one preferred embodiment of the present invention, the coatingmaterial for the layer (C) comprises

-   (C1) at least one radiation-curable binder, preferably selected from    the group consisting of aliphatic urethane (meth)acrylates and    polyester (meth)acrylates,-   (C2) at least one reactive diluent,-   (C3) optionally one or more typical coatings additives selected from    the group of antioxidants, activators (accelerators), extenders,    pigments, dyes, antistats, -, light stabilizers, thickeners,    thixotropic agents, surface-active agents, viscosity modifiers,    plasticizers or chelating agents, preferably selected from the group    consisting of light stabilizers, gloss assistants, and flow control    assistants.

The coating materials for the layer (C) have a double bond density offree-radically polymerizable reactive groups per unit mass of coatingmaterial (based on the sum of components (C1) and (C2)) of at least 1mol/kg, preferably at least 1.5, more preferably at least 2, verypreferably at least 2.5, and more particularly at least 3 mol/kg. Ingeneral a double bond density of not more than 8 mol/kg, preferably notmore than 7, and more preferably not more than 6 mol/kg is sufficient.

In one preferred embodiment, the coating material for the layer (C) hasa viscosity at application temperature in accordance with DIN EN ISO3219 (cone/plate system, shear rate 100 s⁻¹) of 60 to 1200 mPas,preferably of 100 to 300 mPas. Accordingly, the composition, comprisingcomponents (C1) and (C2) and also optionally solvents as flow controlassistants (C3), of this coating material is selected such that thisindicated viscosity is attained.

Binders

The radiation-curable binders in the coating materials may be selectedfor example from the group consisting of

-   -   polyester (meth)acrylates    -   polyether (meth)acrylates    -   urethane (meth)acrylates    -   epoxy (meth)acrylates    -   (meth)acrylated polyacrylates    -   melamine (meth)acrylates or    -   carbonate (meth)acrylates.

Polyester (meth)acrylates are the corresponding esters ofα,β-ethylenically unsaturated carboxylic acids, preferably of(meth)acrylic acid, more preferably of acrylic acid, with polyesterpolyols.

Polyester polyols are known for example from Ullmanns Encyklopadie dertechnischen Chemie, 4th edition, volume 19, pp. 62 to 65. It ispreferred to use polyester polyols obtained by reacting dihydricalcohols with dibasic carboxylic acids. In place of the freepolycarboxylic acids it is also possible to use the correspondingpolycarboxylic anhydrides or corersponding polycarboxylic esters oflower alcohols, or mixtures thereof, for preparing the polyesterpolyols. The polycarboxylic acids may be aliphatic, cycloaliphatic,araliphatic, aromatic or heterocyclic and may optionally be substituted,by halogen atoms for example, and/or unsaturated. Examples thereofinclude the following:

Oxalic acid, maleic acid, fumaric acid, succinic acid, glutaric acid,adipic acid, sebacic acid, dodecanedioic acid, o-phthalic acid,isophthalic acid, terephthalic acid, trimellitic acid, azelaic acid,1,4-cyclohexanedicarboxylic acid or tetrahydrophthalic acid, subericacid, phthalic anhydride, tetrahydrophthalic anhydride,hexahydrophthalic anhydride, tetrachlorophthalic anhydride,endomethylenetetrahydrophthalic anhydride, glutaric anhydride, maleicanhydride, dimeric fatty acids, isomers thereof and hydrogenationproducts thereof, and also esterifiable derivatives, such as anhydridesor dialkyl esters, examples being C1-C4 alkyl esters, preferably methyl,ethyl or n-butyl esters, of the acids stated are used. Preference isgiven to dicarboxylic acids of the general formula HOOC—(CH₂)_(y)—COON,where y is a number from 1 to 20, preferably an even number from 2 to20, more preferably succinic acid, adipic acid, sebacic acid, anddodecanedicarboxylic acid.

Polyhydric alcohols contemplated for preparing the polyesterols include1,2-propanediol, ethylene glycol, 2,2-dimethyl-1,2-ethanediol,1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,3-methylpentane-1,5-diol, 2-ethylhexane-1,3-diol,2,4-diethyloctane-1,3-diol, 1,6-hexanediol, poly THF having a molar massbetween 162 and 2000, poly-1,3-propanediol having a molar mass between134 and 1178, poly-1,2-propanediol having a molar mass between 134 and898, polyethylene glycol having a molar mass between 106 and 458,neopentylglycol, neopentylglycol hydroxypivalate,2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol,2,2-bis(4-hydroxycyclohexyl)propane, 1,1-, 1,2-, 1,3-, and1,4-cyclohexanedimethanol, 1,2-, 1,3- or 1,4-cyclohexanediol,trimethylolbutane, trimethylolpropane, trimethylolethane,neopentylglycol, pentaerythritol, glycerol, ditrimethylolpropane,dipentaerythritol, sorbitol, mannitol, diglycerol, threitol, erythritol,adonitol (ribitol), arabitol (lyxitol), xylitol, dulcitol (galactitol),maltitol or isomalt, which optionally may be alkoxylated as describedabove.

Preferred alcohols are those of the general formula HO—(CH₂)_(x)—OH,where x is a number from 1 to 20, preferably an even number from 2 to20. Preferred are ethylene glycol, butane-1,4-diol, hexane-1,6-diol,octane-1,8-diol, and dodecane-1,12-diol. Further preferred isneopentylglycol.

Also contemplated, furthermore, are polycarbonate diols, of the kindobtainable, for example, by reacting phosgene with an excess of the lowmolecular mass alcohols specified as synthesis components for thepolyester polyols.

Also suitable are lactone-based polyester dials, which are homopolymersor copolymers of lactones, preferably hydroxyl-terminal adducts oflactones with suitable difunctional starter molecules. Lactonescontemplated are preferably those deriving from compounds of the generalformula HO—(CH₂)_(z)—COON, where z is a number from 1 to 20 and whereone H atom in a methylene unit may also be substituted by a C₁ to C₄alkyl radical. Examples are ε-caprolactone, β-propiolactone,gamma-butyrolactone and/or methyl-ε-caprolactone, 4-hydroxybenzoic acid,6-hydroxy-2-naphthoic acid or pivalolactone, and mixtures thereof.Suitable starter components are, for example, the low molecular massdihydric alcohols specified above as a synthesis component for thepolyester polyols. The corresponding polymers of ε-caprolactone areparticularly preferred. Lower polyester diols or polyether diols as wellmay be used as starters for preparing the lactone polymers. In place ofthe polymers of lactones it is also possible to use the corresponding,chemically equivalent polycondensates of the hydroxycarboxylic acidscorresponding to the lactones.

Polyether (meth)acrylates are the corresponding esters ofα,β-ethylenically unsaturated carboxylic acids, preferably of(meth)acrylic acid, more preferably of acrylic acid, with polyetherols.

Preferred in the case of the polyetherols are polyethylene glycol havinga molar mass between 106 and 2000, preferably 106 to 1500, morepreferably 106 to 1000, poly-1,2-propanediol having a molar mass between134 and 1178, poly-1,3-propanediol having a molar mass between 134 and1178, and polytetrahydrofurandiol having a number-average molecularweight M_(n) in the range of about 500 to 4000, preferably 600 to 3000,more particularly 750 to 2000.

Urethane (meth)acrylates are obtainable, for example, by reactingpolyisocyanates with hydroxyalkyl (meth)acrylates or hydroxyalkyl vinylethers and, optionally, chain extenders such as diols, polyols,diamines, polyamines or dithiols or polythiols.

Exemplary urethane (meth)acrylates comprise as synthesis componentssubstantially:

-   (a) at least one organic aliphatic, aromatic or cycloaliphatic di-    or polyisocyanate,-   (b) at least one compound having at least one isocyanate-reactive    group and at least one free-radically polymerizable unsaturated    group,-   (c) optionally, at least one compound having at least two    isocyanate-reactive groups.

For example, aliphatic, aromatic and cycloaliphatic di- andpolyisocyanates having an NCO functionality of at least 1.8, preferably1.8 to 5 and more preferably 2 to 4, and the isocyanurates, biurets,allophanates and uretdiones thereof, are suitable as component (a).

The diisocyanates are preferably isocyanates having 4 to 20 C atoms.Examples of conventional diisocyanates are aliphatic diisocyanates, suchas tetramethylene diisocyanate, hexamethylene diisocyanate(1,6-diisocyanatohexane), octamethylene diisocyanate, decamethylenediisocyanate, dodecamethylene diisocyanate, tetradecamethylenediisocyanate, derivatives of lysine diisocyanate, tetramethylxylylenediisocyanate, trimethylhexane diisocyanate or tetramethylhexanediisocyanate, cycloaliphatic diisocyanates, such as 1,4-, 1,3- or1,2-diisocyanatocyclohexane, 4,4′- or2,4′-di(isocyanatocyclohexyl)methane,1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophoronediisocyanate), 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane or 2,4- or2,6-diisocyanato-1-methylcyclohexane, and also 3 (or 4), 8 (or9)-bis(isocyanatomethyl)tricycle[5.2.1.0^(2,6)]decane isomer mixtures,and aromatic diisocyanates, such as toluene 2,4- or 2,6-diisocyanate andthe isomer mixtures thereof, m- or p-xylylene diisocyanate, 2,4′- or4,4′-diisocyanatodiphenylmethane and the isomer mixtures thereof,phenylene 1,3- or 1,4-diisocyanate, 1-chlorophenylene 2,4-diisocyanate,naphthylene 1,5-diisocyanate, biphenylene 4,4′-diisocyanate,4,4′-diisocyanato-3,3′-dimethylbiphenyl, 3-methyldiphenylmethane4,4′-diisocyanate, tetramethylxylylene diisocyanate,1,4-diisocyanatobenzene or 4,4′-diisocyanatodiphenyl ether.

Mixtures of said diisocyanates may also be present.

Hexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,isophorone diisocyanate and di(isocyanatocyclohexyl)methane arepreferred.

Depending on whether the diisocyanate on which the polyisocyanate isbased is aromatic or nonaromatic, the resultant urethane (meth)acrylateis referred to as an aromatic urethane (meth)acrylate or as an aliphaticurethane (meth)acrylate.

Suitable polyisocyanates are polyisocyanates having isocyanurate groups,uretdione diisocyanates and polyisocyanates having biuret groups,polyisocyanates having urethane or allophanate groups, polyisocyanatescomprising oxadiazinetrione groups, uretonimine-modified polyisocyanatesof linear or branched C₄-C₂₀ alkylene diisocyanates, cycloaliphaticdiisocyanates having in total 6 to 20 C atoms or aromatic diisocyanateshaving in total 8 to 20 C atoms or mixtures thereof.

The di- and polyisocyanates which can be used preferably contain from10% to 60% by weight, based on the di- and polyisocyanate (mixture),preferably from 15% to 60% by weight and particularly preferably from20% to 55% by weight, of isocyanate groups (calculated as NCO, molecularweight=42).

Aliphatic or cycloaliphatic di- and polyisocyanates, for example theabovementioned aliphatic or cycloaliphatic diisocyanates, or mixturesthereof, are preferred.

The following are furthermore preferred:

-   1) Polyisocyanates having isocyanurate groups and obtained from    aromatic, aliphatic and/or cycloaliphatic diisocyanates.    Particularly preferred here are the corresponding aliphatic and/or    cycloaliphatic isocyanatoisocyanurates and in particular those based    on hexamethylene diisocyanate and isophorone diisocyanate. The    isocyanurates present are in particular trisisocyanatoalkyl or    trisisocyanatocycloalkyl isocyanurates, which are cyclic trimers of    the diisocyanates, or are mixtures with their higher homologs having    more than one isocyanurate ring. The isocyanatoisocyanurates    generally have an NCO content of from 10% to 30% by weight, in    particular from 15% to 25% by weight, and an average NCO    functionality of from 3 to 4.5.-   2) Uretdione diisocyanates having aromatically, aliphatically and/or    cycloaliphatically bonded isocyanate groups, preferably having    aliphatically and/or cycloaliphatically bonded groups and in    particular those derived from hexamethylene diisocyanate or    isophorone diisocyanate. Uretdione diisocyanates are cyclic    dimerization products of diisocyanates.    -   The uretdione diisocyanates can be used in the formulations as a        sole component or as a mixture with other polyisocyanates, in        particular those mentioned under 1).-   3) Polyisocyanates having biuret groups and having aromatically,    cycloaliphatically or aliphatically bonded, preferably    cycloaliphatically or aliphatically bonded, isocyanate groups, in    particular tris(6-isocyanatohexyl)biuret or mixtures thereof with    its higher homologs. These polyisocyanates having biuret groups    generally have an NCO content of from 18% to 22% by weight and an    average NCO functionality of from 3 to 4.5.-   4) Polyisocyanates having urethane and/or allophanate groups and    having aromatically, aliphatically or cycloaliphatically bonded,    preferably aliphatically or cycloaliphatically bonded, isocyanate    groups, as can be obtained, for example, by reaction of excess    amounts of hexamethylene diisocyanate or of isophorone diisocyanate    with polyhydric alcohols, such as, for example, trimethylolpropane,    neopentyl glycol, pentaerythritol, 1,4-butanediol, 1,6-hexanediol,    1,3-propanediol, ethylene glycol, diethylene glycol, glycerol,    1,2-dihydroxypropane or mixtures thereof. These polyisocyanates    having urethane and/or allophanate groups generally have an NCO    content of from 12% to 20% by weight and an average NCO    functionality of from 1.8 to 3.-   5) Polyisocyanates comprising oxadiazinetrione groups, preferably    derived from hexamethylene diisocyanate or isophorone diisocyanate.    Such polyisocyanates comprising oxadiazinetrione groups can be    prepared from diisocyanate and carbon dioxide.-   6) Uretonimine-modified polyisocyanates.

The polyisocyanates 1) to 6) can be used as a mixture, optionally alsoas a mixture with diisocyanates.

Compounds suitable as component (b) are those which carry at least oneisocyanate-reactive group and at least one free-radically polymerizablegroup.

Groups reactive toward isocyanate may be, for example, —OH, —SH, —NH₂and —NHR′, where R′ is hydrogen or an alkyl group comprising 1 to 4carbon atoms, such as, for example, methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl, sec-butyl or tert-butyl.

Components (b) may be, for example, monoesters of α,β-unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, crotonic acid,itaconic acid, fumaric acid, maleic acid, acrylamidoglycolic acid ormethacrylamidoglycolic acid, or vinyl ethers with di- or polyols, whichpreferably have 2 to 20 carbon atoms and at least two hydroxyl groups,such as ethylene glycol, diethylene glycol, triethylene glycol,1,2-propylene glycol, 1,3-propylene glycol, 1,1-dimethyl-1,2-ethanediol,dipropylene glycol, triethylene glycol, tetraethylene glycol,pentaethylene glycol, tripropylene glycol, 1,4-butanediol,1,5-pentanediol, neopentylglycol, 1,6-hexanediol,2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,1,4-dimethylolcyclohexane, 2,2-bis(4-hydroxycyclohexyl)propane,glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol, ditrimethylolpropane, erythritol, sorbitol, poly THFhaving a molar weight of between 162 and 2000, poly-1,3-propanediolhaving a molar weight of between 134 and 400 or polyethylene glycolhaving a molar weight of between 238 and 458. It is furthermore possibleto use esters or amides of (meth)acrylic acid with amino alcohols, e.g.2-aminoethanol, 2-(methylamino)ethanol, 3-amino-1-propanol,1-amino-2-propanol or 2-(2-aminoethoxyl)ethanol, 2-mercaptoethanol orpolyaminoalkanes, such as ethylenediamine or diethylenetriamine, orvinylacetic acid.

Unsaturated polyetherols or polyesterols or polyacrylate polyols havingan average OH functionality of 2 to 10 are furthermore suitable.

Examples of amides of ethylenically unsaturated carboxylic acids withamino alcohols are hydroxyalkyl(meth)acrylamides, such asN-hydroxymethylacrylamide, N-hydroxymethylmethacrylamide,N-hydroxyethylacrylamide, N-hydroxyethylmethacrylamide,5-hydroxy-3-oxapentyl(meth)acrylamide, N-hydroxyalkylcrotonamides, suchas N-hydroxymethylcrotonamide or N-hydroxyalkylmaleimides, such asN-hydroxyethylmaleimide.

2-Hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate,1,4-butanediol mono(meth)acrylate, neopentylglycol mono(meth)acrylate,1,5-pentanediol mono(meth)acrylate, 1,6-hexanediol mono(meth)acrylate,glyceryl mono- and di(meth)acrylate, trimethylolpropane mono- anddi(meth)acrylate, pentaerythrityl mono-, di- and tri(meth)acrylate, and4-hydroxybutyl vinyl ether, 2-aminoethyl (meth)acrylate, 2-aminopropyl(meth)acrylate, 3-aminopropyl (meth)acrylate, 4-aminobutyl(meth)acrylate, 6-aminohexyl (meth)acrylate, 2-thioethyl (meth)acrylate,2-aminoethyl(meth)acrylamide, 2-aminopropyl(meth)acrylamide,3-aminopropyl-(meth)acrylamide, 2-hydroxyethyl(meth)acrylamide,2-hydroxypropyl(meth)acrylamide or 3-hydroxypropyl(meth)acrylamide arepreferably used. 2-Hydroxyethyl acrylate, 2-hydroxyethyl methacrylate,2- or 3-hydroxypropyl acrylate, 1,4-butanediol monoacrylate and3-(acryloyloxy)-2-hydroxypropyl methacrylate are particularly preferred.

Compounds which are suitable as component (c) are those which have atleast two groups reactive toward isocyanate, for example —OH, —SH, —NH₂or —NHR², where R² therein, independently of one another, may behydrogen, methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl,sec-butyl or tert-butyl.

These are preferably diols or polyols, such as hydrocarbondiols having 2to 20 carbon atoms, e.g. ethylene glycol, 1,2-propanediol,1,3-propanediol, 1,1-dimethylethane-1,2-diol, 1,6-hexanediol,1,10-decanediol, bis(4-hydroxycyclohexane)isopropylidene,tetramethylcyclobutanediol, 1,2-, 1,3- or 1,4-cyclohexanediol,cyclooctanediol, norbornanediol, pinanediol, decalindiol, etc., estersthereof with short-chain dicarboxylic acids, such as adipic acid orcyclohexanedicarboxylic acid, carbonates thereof, prepared by reactionof the diols with phosgene or by transesterification with dialkyl ordiaryl carbonates, or aliphatic diamines, such as methylene- andisopropylidenebis(cyclohexylamine), piperazine, 1,2-, 1,3- or1,4-diaminocyclohexane, 1,2-, 1,3- or 1,4-cyclohexanebis(methylamine),etc., dithiols or polyfunctional alcohols, secondary or primary aminoalcohols, such as ethanolamine, diethanolamine, monopropanolamine,dipropanolamine, etc., or thioalcohols, such as thioethylene glycol.

Diethylene glycol, triethylene glycol, dipropylene glycol, tripropyleneglycol, neopentyl glycol, pentaerythritol, 1,2- and 1,4-butanediol,1,5-pentanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, 1,2-,1,3- and 1,4-dimethylolcyclohexane, 2,2-bis(4-hydroxycyclohexyl)propane,glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane,dipentaerythritol, ditrimethylolpropane, erythritol and sorbitol,2-aminoethanol, 3-amino-1-propanol, 1-amino-2-propanol or2-(2-aminoethoxyl)ethanol, bisphenol A or butanetriol are furthermoreconceivable.

Unsaturated polyetherols or polyesterols or polyacrylate polyols havingan average OH functionality of 2 to 10, and polyamines, such as, forexample, polyethylenimine, or polymers of, for example,poly-N-vinylformamide which comprise free amine groups, are furthermoresuitable.

The cycloaliphatic diols, such as, for example,bis(4-hydroxycyclohexane)isopropylidene, tetramethylcyclobutanediol,1,2-, 1,3- or 1,4-cyclohexanediol, cyclooctanediol or norbornanediol,are particularly suitable here.

The urethane (meth)acrylates which can be used by way of example areobtained by reacting the components (a), (b) and (c) with one another.

The molar composition (a):(b):(c) per 3 mol of reactive isocyanategroups in (a) here is generally as follows:

-   (b) 0.5-3.0, preferably 0.8-2.5, particularly preferably 1.0-2.2 and    in particular 1.4-1.8 mol of groups reactive toward isocyanate and-   (c) 0-2.0, preferably 0.1-1.8, particularly preferably 0.5-1.5 and    in particular 0.8-1.3 mol of groups reactive toward isocyanate.

The formation of the adduct from the compound containing isocyanategroups and the compound which comprises groups reactive towardisocyanate groups is effected, as a rule, by mixing the components inany desired sequence, optionally at elevated temperature.

Preferably, the compound which comprises groups reactive towardisocyanate groups is added to the compound containing isocyanate groups,preferably in a plurality of steps.

Particularly preferably, the compound containing isocyanate groups isinitially taken and the compounds which comprise groups reactive towardisocyanate are added. In particular, the compound (a) containingisocyanate groups is initially taken and then (b) is added. Optionally,desired further components can subsequently be added.

As a rule, the reaction is carried out at temperatures between 5 and100° C., preferably between 20 to 90° C. and particularly preferablybetween 40 and 80° C. and in particular between 60 and 80° C.

The procedure is preferably carried out under anhydrous conditions.

Here, anhydrous means that the water content in the reaction system isnot more than 5% by weight, preferably not more than 3% by weight andparticularly preferably not more than 1% by weight.

In order to suppress polymerization of the polymerizable double bonds,the procedure is preferably carried out under an oxygen-containing gas,particularly preferably air or air-nitrogen mixtures.

Air or a mixture of oxygen or air and a gas which is inert under theconditions of use can preferably be used as the oxygen-containing gas.Nitrogen, helium, argon, carbon monoxide, carbon dioxide, steam, lowerhydrocarbons or mixtures thereof can be used as the inert gas.

The oxygen content of the oxygen-containing gas may be, for example,between 0.1% and 22% by volume, preferably from 0.5 to 20, particularlypreferably from 1 to 15, very particularly preferably from 2 to 10 andin particular from 4 to 10% by volume. If desired, higher oxygencontents can of course also be used.

The reaction can also be carried out in the presence of an inertsolvent, e.g. acetone, isobutyl methyl ketone, toluene, xylene, butylacetate or ethoxyethyl acetate. However, the reaction is preferablycarried out in the absence of a solvent.

The urethane (meth)acrylates preferably have a number average molarweight M_(n) of at least 500 and preferably at least 1000 g/mol. Theypreferably have a number average molar weight M_(n) of up to 20 000,particularly preferably up to 10 000 and very particularly preferably upto 4000, g/mol (determined by gel permeation chromatography usingtetrahydrofuran and polystyrene as standard).

The urethane (meth)acrylates preferably contain from 1 to 5,particularly preferably from 2 to 4, mol of (meth)acrylic groups per1000 g of urethane (meth)acrylate.

The urethane vinyl ethers preferably contain from 1 to 5, particularlypreferably from 2 to 4, mol of vinyl ether groups per 1000 g of urethanevinyl ether.

Epoxide (meth)acrylates are obtainable by reacting epoxides with(meth)acrylic acid. Suitable epoxides are, for example, epoxidizedolefins, aromatic glycidyl ethers or aliphatic glycidyl ethers,preferably those of aromatic or aliphatic glycidyl ethers.

Epoxidized olefins may be, for example, ethylene oxide, propylene oxide,isobutylene oxide, 1-butene oxide, 2-butene oxide, vinyloxirane, styreneoxide or epichlorohydrin, preferably ethylene oxide, propylene oxide,isobutylene oxide, vinyloxirane, styrene oxide or epichlorohydrin,particularly preferably ethylene oxide, propylene oxide orepichlorohydrin and very particularly preferably ethylene oxide andepichlorohydrin.

Aromatic glycidyl ethers are, for example, bisphenol A diglycidyl ether,bisphenol F diglycidyl ether, bisphenol B diglycidyl ether, bisphenol Sdiglycidyl ether, hydroquinone diglycidyl ether, alkylation products ofphenol/dicyclopentadiene, e.g.2,5-bis[(2,3-epoxypropoxy)phenyl]octahydro-4,7-methano-5H-indene (CASNo. [13446-85-0]), tris[4-(2,3-epoxypropoxy)phenyl]methane isomers (CASNo. [66072-39-7]), phenol-based epoxy novolaks (CAS No. [9003-35-4]) andcresol-based epoxy novolaks (CAS No. [37382-79-9]).

Aliphatic glycidyl ethers are, for example, 1,4-butanediol diglycidylether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidylether, pentaerythrityl tetraglycidyl ether,1,1,2,2-tetrakis[4-(2,3-epoxypropoxyl)phenyl]ethane (CAS No.[27043-37-4]), diglycidyl ethers of polypropylene glycol(α,ω-bis(2,3-epoxypropoxy)poly(oxypropylene)) (CAS No. [16096-30-3]) andof hydrogenated bisphenol A(2,2-bis[4-(2,3-epoxypropoxyl)cyclohexyl]propane) (CAS No.[13410-58-7]).

The epoxide (meth)acrylates and epoxide vinyl ethers preferably have anumber average molar weight M_(n) of from 200 to 20 000, particularlypreferably from 200 to 10 000, g/mol and very particularly preferablyfrom 250 to 3000 g/mol; the content of (meth)acrylic and vinyl ethergroups is preferably from 1 to 5, particularly preferably from 2 to 4,per 1000 g of epoxide (meth)acrylate or vinyl ether epoxide (determinedby gel permeation chromatography using polystyrene as a standard andtetrahydrofuran as an eluent).

(Meth)acrylated polyacrylates are the corresponding esters ofα,β-ethylenically unsaturated carboxylic acids, preferably of(meth)acrylic acid, more preferably of acrylic acid, with polyacrylatepolyols.

Such polyacrylate polyols preferably have a molecular weight M_(n) of atleast 1000, more preferably at least 2000, and very preferably at least5000 g/mol. The molecular weight M_(n) may be for example up to 200 000,preferably up to 100 000, more preferably up to 80 000, and verypreferably up to 50 000 g/mol.

Preferred OH numbers for the polyacrylate polyols, measured inaccordance with DIN 53240-2, are 15-250 mg KOH/g, preferably 80-160 mgKOH/g.

Additionally the polyacrylate polyols may have an acid number to DIN ENISO 3682 of up to 200 mg KOH/g, preferably up to 150 and more preferablyup to 100 mg KOH/g.

The polyacrylate polyols are copolymers of at least one (meth)acrylicester having at least one compound having at least one, preferablyexactly one, hydroxyl group and at least one, preferably exactly one,(meth)acrylate group.

The latter may be, for example, monoesters of α,β-unsaturated carboxylicacids, such as acrylic acid, methacrylic acid (referred to for short inthis specification as “(meth)acrylic acid”), with diols or polyols,which preferably have 2 to 20 C atoms and have at least two hydroxylgroups, such as ethylene glycol, diethylene glycol, triethylene glycol,1,2-propylene glycol, 1,3-propylene glycol, 1,1-dimethyl-1,2-ethanediol,dipropylene glycol, triethylene glycol, tetraethylene glycol,pentaethylene glycol, tripropylene glycol, 1,4-butanediol,1,5-pentanediol, neopentylglycol, neopentylglycol hydroxypivalate,2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol,2-butyl-2-ethyl-1,3-propanediol, 1,6-hexanediol,2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,2-ethyl-1,3-hexanediol, 2,4-diethyloctane-1,3-diol,2,2-bis(4-hydroxycyclohexyl)propane, 1,1-, 1,2-, 1,3-, and1,4-bis(hydroxymethyl)cyclohexane, 1,2-, 1,3- or 1,4-cyclohexanediol,glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol, ditrimethylolpropane, dipentarythritol, sorbitol,mannitol, diglycerol, threitol, erythritol, adonitol (ribitol), arabitol(lyxitol), xylitol, dulcitol (galactitol), maltitol, isomalt, polyTHFhaving a molar weight between 162 and 4500, preferably 250 to 2000,poly-1,3-propanediol or polypropylene glycol having a molar weightbetween 134 and 2000, or polyethylene glycol having a molar weightbetween 238 and 2000.

Preference is given to 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 2- or 3-hydroxypropyl acrylate, 1,4-butanediolmonoacrylate or 3-(acryloyloxy)-2-hydroxypropyl acrylate, and morepreferably 2-hydroxyethyl acrylate and/or 2-hydroxyethyl methacrylate.

The hydroxyl-bearing monomers are used in the copolymerization in amixture with other polymerizable monomers, preferably free-radicallypolymerizable monomers, preferably those consisting to an extent of morethan 50% by weight of C₁-C₂₀, preferably C₁ to C₄ alkyl (meth)acrylate,(meth)acrylic acid, vinylaromatics having up to 20 C atoms, vinyl estersof carboxylic acids comprising up to 20 C atoms, vinyl halides,nonaromatic hydrocarbons having 4 to 8 C atoms and 1 or 2 double bonds,unsaturated nitriles, and mixtures thereof. Particularly preferred arethe polymers which consist to an extent of more than 60% by weight ofC₁-C₁₀ alkyl (meth)acrylates, styrene, vinylimidazole or mixturesthereof.

The polymers may further comprise hydroxy-functional monomers in linewith the above hydroxyl group content, and optionally further monomers,examples being (meth)acrylic acid glycidyl epoxy esters, ethylenicallyunsaturated acids, more particularly carboxylic acids, acid anhydridesor acid amides.

Examples of suitable ethylenically unsaturated melamine resins are thereaction products of melamine/formaldehyde condensates withOH-containing, ethylenically unsaturated compounds, ethylenicallyunsaturated dicarboxylic anhydrides, or with the amides of ethylenicallyunsaturated monocarboxylic acids. Suitable melamine/formaldehydecondensates are more particularly hexamethylolmelamine (HMM) andhexamethoxymethylolmelamine (HMMM). Examples of suitable OH-containingcompounds include the hydroxyalkyl esters of ethylenically unsaturatedcarboxylic acids, particularly of acrylic acid and of methacrylic acid.Additionally suitable for the reaction with HMM are ethylenicallyunsaturated alcohols, such as allyl alcohol or crotyl alcohol, orethylenically unsaturated dicarboxylic anhydrides, such as maleicanhydride. Furthermore, it is possible to modify not only HMM but alsoHMMM with the amides of ethylenically unsaturated carboxylic acids,examples being acrylamide or methacrylamide, to give ethylenicallyunsaturated melamine resins. Melamine resins of these kinds are known tothe skilled person and described in, for example, P.K.T. Oldring, p. 208to p. 214, and also in EP-A 464 466 and DE-A 25 50 740, herebyincorporated by reference for further details.

Carbonate (meth)acrylates are likewise obtainable with variousfunctionalities.

The number-average molecular weight M_(n) of the carbonate(meth)acrylates is preferably less than 3000 g/mol, more preferably lessthan 1500 g/mol, more preferably less than 800 g/mol (determined by gelpermeation chromatography using polystyrene as a standard andtetrahydrofuran as a solvent).

The carbonate (meth)acrylates are obtainable in a simple manner bytransesterification of carbonic esters with polyhydric, preferablydihydric, alcohols (diols, e.g. hexanediol) and subsequentesterification of the free OH groups with (meth)acrylic acid, ortransesterification with (meth)acrylic esters, as described, forexample, in EP-A 92 269. They are also obtainable by reacting phosgene,urea derivatives with polyhydric, e.g. dihydric, alcohols.

In an analogous manner, vinyl ether carbonates are also obtainable byreacting a hydroxyalkyl vinyl ether with carbonic esters and,optionally, dihydric alcohols.

(Meth)acrylates or vinyl ethers of polycarbonatepolyols, such as thereaction product of one of said di- or polyols and a carbonic ester andof a hydroxyl-containing (meth)acrylate or vinyl ether, are alsoconceivable.

Suitable carbonic esters are, for example, ethylene or 1,2- or1,3-propylene carbonate or dimethyl, diethyl or dibutyl carbonate.

Suitable hydroxyl-containing (meth)acrylates are, for example,2-hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate,1,4-butanediol mono(meth)acrylate, neopentylglycol mono(meth)acrylate,glyceryl mono- and di(meth)acrylate, trimethylolpropane mono- anddi(meth)acrylate and pentaerythrityl mono-, di- and tri(meth)acrylate.

Suitable hydroxyl-containing vinyl ethers are, for example,2-hydroxyethyl vinyl ether and 4-hydroxybutyl vinyl ether.

Particularly preferred carbonate (meth)acrylates are those of theformula:

where R is H or CH₃, X is a C₂-C₁₈ alkylene group and n is an integerfrom 1 to 5, preferably from 1 to 3.

R is preferably H and X is preferably C₂ to C₁₀ alkylene, for example1,2-ethylene, 1,2-propylene, 1,3-propylene, 1,4-butylene or1,6-hexylene, particularly preferably C4 to C8 alkylene. Veryparticularly preferably, X is C6 alkylene.

The carbonate (meth)acrylates are preferably aliphatic carbonate(meth)acrylates.

Reactive Diluents

Reactive diluents in the context of the present specification are thosecompounds having in general one to four free-radically polymerizablereactive groups which at 23° C. have a viscosity in accordance with DINEN ISO 3219 (cone/plate system, shear rate 100 s⁻¹) below 150 mPas.

In accordance with the present specification, the reactive diluentspreferably are selected from the group consisting of polyfunctional(meth)acrylic esters and monofunctional (meth)acrylic esters, morepreferably from such (meth)acrylic esters having a molar weight of up to350 g/mol.

Polyfunctional (meth)acrylic esters may be, for example, esters ofα,β-ethylenically unsaturated carboxylic esters, preferably of(meth)acrylic acid, more preferably of acrylic acid, withcorrespondingly at least difunctional polyalcohols.

Polyalcohols of this kind are, for example, at least dihydric polyols,polyetherols or polyesterols, or polyacrylate polyols, having an averageOH functionality of at least 2, preferably 3 to 10, suitably.

Examples of polyfunctional, polymerizable compounds are ethylene glycoldiacrylate, 1,2-propanediol diacrylate, 1,3-propanediol diacrylate,1,4-butanediol diacrylate, 1,3-butanediol diacrylate, 1,5-pentanedioldiacrylate, 1,6-hexanediol diacrylate, 1,8-octanediol diacrylate,neopentylglycol diacrylate, 1,1-, 1,2-, 1,3-, and1,4-cyclohexanedimethanol diacrylate, and 1,2-, 1,3- or1,4-cyclohexanediol diacrylate.

Other examples of polyfunctional polymerizable compounds aretrimethylolpropane triacrylate, ditrimethylolpropane pentaacrylate orhexaacrylate, pentaerythritol triacrylate or tetraacrylate, glyceroldiacrylate or triacrylate, and also diacrylates and polyacrylates ofsugar alcohols, such as, for example, sorbitol, mannitol, diglycerol,threitol, erythritol, adonitol (ribitol), arabitol (lyxitol), xylitol,dulcitol (galactitol), maltitol or isomalt.

Further examples thereof are (meth)acrylates of compounds of the formula(IIa) to (IId),

in whichR⁵ and R⁶ independently of one another are hydrogen or are C₁-C₁₈ alkyloptionally substituted by aryl, alkyl, aryloxy, alkyloxy, heteroatomsand/or heterocycles,u, v, w, and x independently of one another are each an integer from 1to 10, preferably 1 to 5, and more preferably 1 to 3, and

each X_(i) for i=1 to u, 1 to v, 1 to w, and 1 to x, independently ofone another, may be selected from the group —CH₂—CH₂—O—,—CH₂—CH(CH₃)—O—, —CH(CH₃)—CH₂—O—, —CH₂—C(CH₃)₂—O—, —C(CH₃)₂—CH₂—O—,—CH₂—CHVin-O—, —CHVin-CH₂—O—, —CH₂—CHPh-O— and —CHPh-CH₂—O—, preferablyfrom the group —CH₂—CH₂—O—, —CH₂—CH(CH₃)—O—, and —CH(CH₃)—CH₂—O—, andmore preferably —CH₂—CH₂—O—,

in which Ph stands for phenyl and Vin stands for vinyl.

Herein, C₁-C₁₈ alkyl optionally substituted by aryl, alkyl, aryloxy,alkyloxy, heteroatoms and/or heterocycles means, for example, methyl,ethyl, propyl, isopropyl, n-butyl, sec-butyl, tent-butyl, pentyl, hexyl,heptyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl, decyl, dodecyl,tetradecyl, hexadecyl, octadecyl, 1,1-dimethylpropyl, 1,1-dimethylbutyl,1,1,3,3-tetramethylbutyl, preferably methyl, ethyl or n-propyl, verypreferably methyl or ethyl.

Preferably these are (meth)acrylates of singly to vigintuply and morepreferably triply to decuply ethoxylated, propoxylated or mixedlyethoxylated and propoxylated, and more particualrly exclusivelyethoxylated, neopentylglycol, trimethylolpropane, trimethylolethane orpentaerythritol.

Preferred polyfunctional polymerizable compounds are ethylene glycoldiacrylate, 1,2-propanediol diacrylate, 1,3-propanediol diacrylate,1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, trimethylolpropanetriacrylate, pentaerythritol tetraacrylate, and tricrylate of singly tovigintuply alkoxylated, more preferably ethoxylated, trimethylolpropane.

Especially preferred polyfunctional polymerizable compounds are1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, trimethylolpropanetriacrylate, pentaerythritol tetraacrylate, and triacrylate of singly tovigintuply ethoxylated trimethylolpropane.

Monofunctional (meth)acrylic esters are those having exactly one(meth)acrylic acid group. Examples thereof are alkyl (meth)acrylates,cycloalkyl (meth)acrylates, and (meth)acrylates of heterocycliccompounds.

Examples of alkyl (meth)acrylates are esters of (meth)acrylic acid withalcohols having 1 to 20 C atoms, preferably C1-C20 alkyl(meth)acrylates, more preferably selected from the group consisting ofmethyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, and2-ethylhexyl (meth)acrylate.

The alcohols may also be substituted, as is the case, for example, with2-phenoxyethyl acrylate or 2-((2′-ethoxy)ethoxy)ethyl acrylate.

Examples of cycloalkyl (meth)acrylates are esters of (meth)acrylic acidwith cycloalkanols or bicycloalkanols, the cycloalkanol orbicycloalkanol having from 3 to 20 carbon atoms, preferably 5 to 10carbon atoms, and being able to be optionally substituted by C₁- to C₄alkyl.

Examples of cycloalkanol and bicycloalkanol are cyclopentanol,cyclohexanol, cyclooctanol, cyclododecanol, 4-methylcyclohexanol,4-isopropylcyclohexanol, 4-tert-butylcyclohexanol (preferably in cisconfiguration), dihydrodicyclopentadienyl alcohol, isoborneol, andnorbornyl alcohol. Preference is given to isoborneol, cyclohexanol, and4-tert-butylcyclohexanol.

(Meth)acrylates of heterocyclic compounds are monofunctional esters ofα,β-ethylenically unsaturated carboxylic acids, preferably of acrylicacid or methacrylic acid, with a monofunctional alcohol which has atleast one saturated 5- or 6-membered heterocycle having one or twooxygen atoms in the ring as a structural element.

Examples thereof can be found in WO 2010/121978 A1, page 12 lines 12 37,hereby incorporated by reference to be part of the present disclosurecontent.

Preferred (meth)acrylates of heterocyclic compounds aretrimethylolpropane monoformal acrylate, glycerol monoformal acrylate,4-tetrahydropyranyl acrylate, 2-tetrahydropyranyl methylacrylate,tetrahydrofurfuryl acrylate, and mixtures thereof.

Application

The coating of the light alloy rims with the coating materials describedtakes place in accordance with customary methods that are known to theskilled person, in which a coating material or a surface-coatingformulation comprising it is applied to the substrate to be coated inthe desired thickness and optionally is dried and/or optionally ispartly or fully cured. This operation may be repeated one or more timesif desired. Application to the substrate may be accomplished in a knownway, as for example by dipping, spraying, trowelling, knifecoating,brushing, rolling, rollercoating, pouring, laminating, injectionbackmolding or coextruding, preferably by dipping or spraying, morepreferably by spraying.

The coating compositions may be applied by any of a very wide variety ofspraying methods, one or more times, such as air-pressure, airless orelectrostatic spraying methods, for example. The coating thickness issituated generally within a range from about 3 to 1000 g/m² andpreferably 10 to 200 g/m².

Application, drying and curing of the coatings take place in generalunder standard temperature conditions, i.e., without the coating beingheated, but may also be applied at elevated temperature (see nextparagraph). Alternatively the coating materials may be used forproducing coatings which following application are dried and cured atelevated temperature, e.g., at 40-250° C., preferably 40-150° C., andmore particularly at 40 to 100° C. This is limited by the thermalstability of the substrate.

In one preferred embodiment the coating material is applied with anelevated temperature, as for example 30 to 80, preferably 40 to 60, °C., with the substrate being at a lower temperature. The advantage ofthis is that the coating material has a lower viscosity on application,as a result of the higher temperature, and the cooling on the substrateenhances the holdout properties (low tendency toward sagging).

Curing

Radiation curing takes place using high-energy light, as for example UVlight, or electron beams, preferably UV light. Radiation curing may takeplace at relatively high temperatures. For example, a temperature abovethe T_(g) of the radiation-curable binder may be advisable here.

Radiation curing here means the free-radical polymerization ofpolymerizable compounds as a consequence of electromagnetic and/orparticulate radiation, preferably UV light in the wavelength range ofA=200 to 700 nm and/or electron beams in the range from 150 to 300 keV,and more preferably with a radiation dose of at least 80, preferably 80to 3000 mJ/cm².

Drying and curing of the coatings take place in general under standardtemperature conditions, i.e., without the coating being heated.Alternatively the coating materials may also be used for producingcoatings which following application are dried and cured at elevatedtemperature, e.g., at 40-150° C., preferably 40-100° C., and moreparticularly at 40 to 80° C.

Further disclosed is a method for coating substrates by applying to thesubstrate the coating materials described, or surface-coatingformulations comprising them, optionally admixed with thermally curableresins, drying the applied coating, and then curing it with electronbeams or UV exposure in an oxygen-containing atmosphere or, preferably,under inert gas, optionally at temperatures up to the level of thedrying temperature.

It is possible optionally, if two or more layers of the coatingcomposition are applied one above another, to carry out drying and/orradiation curing after each coating operation.

Examples of suitable radiation sources for the radiation cure arelow-pressure, medium-pressure, and high-pressure mercury lamps, and alsofluorescent tubes, pulsed lamps, metal halide lamps, electronic flashdevices which allow radiation curing without a photoinitiator, orexcimer emitters. The radiation cure is accomplished by exposure tohigh-energy radiation, i.e., UV radiation or daylight, preferably lightin the wavelength range of λ=200 to 700 nm, more preferably of λ=200 to500 nm, and very preferably λ=250 to 400 nm, or by bombardment withhigh-energy electrons (electron beams; 150 to 300 keV). Examples ofradiation sources used include high-pressure mercury vapor lamps,lasers, pulsed lamps (flash light), halogen lamps, LED lamps or excimeremitters. The radiation dose normally sufficient for crosslinking in thecase of UV curing is situated in the range from 80 to 3000 mJ/cm².

It is of course also possible to use two or more radiation sources forthe cure—two to four, for example.

These sources may also each emit in different wavelength ranges.

Since the light alloy rims are covered on all sides with coatingmaterials, at least with the layer (A), curing also takes place from thesides coated.

Irradiation may with preference also be carried out in the absence ofoxygen, as under an inert gas atmosphere, for example. Suitable inertgases are preferably nitrogen, noble gases, carbon dioxide, orcombustion gases. Irradiation may take place, furthermore, with thecoating material covered with transparent media. Examples of transparentmedia are polymeric films, glass or liquids, water for example.Particular preference is given to irradiation in the manner described inDE-A1 199 57 900.

If curing takes place by means of UV radiation rather than electronbeams, then it is self-evident that the radiation-curable coatingmaterial comprises in each case at least one photoinitiator which can beactivated in a wavelength range of the irradiated radiation, and forwhich the ingredients present in the coating materials are transparent.

Suitable photoinitiators include those as described in WO 2006/005491A1, page 21 line 18 to page 22 line 2 (corresponding to US 2006/0009589A1, paragraph [0150]), hereby incorporated by reference to become partof the present disclosure content.

As an example, an illustration may be given of a method for coatinglight alloy rims that comprises at least the following steps:

providing a mechanically prepared rim body,coating the rim body with at least one conversion coating, followed bydrying,coating with at least one coating material which comprises at least oneradiation-curable compound and optionally at least one anticorrosionpigment and/or corrosion inhibitor, the coating material having an acidnumber of 10 to 120 mg KOH/g,optionally flashing the resultant coating (A1),coating with at least one coating material which comprises at least oneradiation-curablecompound and at least one anticorrosion pigment and/or corrosioninhibitor,subsequently, at least partial curing by radiation, followed bycoating with at least one water-based, two-component polyurethanecoating material, comprising at least one pigment and/or metallicflakes,subsequently flashing and drying of the coating, as for example at 60 to100° C. over a period of 10 to 30 minutes,coating with at least one radiation-curable coating material comprisingat least one radiation-curable compound and optionally one or moretypical coatings additives,followed by complete curing of the surface-coating layers by radiation.

As an example, an illustration may be given of a method for coatinglight alloy rims that comprises at least the following steps:

providing a mechanically prepared rim body,coating the rim body with at least one conversion coating, followed bydrying,coating with at least one coating material which comprises at least oneradiation-curable compound and optionally at least one anticorrosionpigment and/or corrosion inhibitor, the coating material having an acidnumber of 10 to 120 mg KOH/g,optionally flashing the resultant coating (A1),coating with at least one coating material which comprises at least oneradiation-curable compound and at least one anticorrosion pigment and/orcorrosion inhibitor,subsequently, at least partial curing by radiation, followed bycoating with at least one water-based, radiation-curable coatingmaterial, comprising at least one pigment and/or metallic flakes,subsequently, at least partial curing by radiation,coating with at least one radiation-curable coating material comprisingat least one radiation-curable compound and optionally one or moretypical coatings additives,followed by complete curing of the surface-coating layers by radiation.

EXAMPLES Example 1 Adhesion Primer A1 (1^(st) Layer Directly onSubstrate)

Proportion in the Trade formulation Function in the name Chemicaldescription [%] Component formulation Difunctional aliphatic urethane28.0 A1a Binder for film acrylate, having a molecular weight forming ofabout 1500 g/mol, diluted in trimethylolpropane formal monoacrylate4-tert-Butylcyclohexyl acrylate 39.0 A1b Low-shrink reactive diluentPreparation based on: 10.0 A1b Acidic 2-(phosphonooxy)ethyl adhesionmethacrylate, promoter bis(methacryloyloxyethyl)hydrogenphosphateLugalvan ® 1-Benzyl-3-carboxylatopyridinium 1.0 A1c Corrosion BPC 48inhibitor Shieldex ® Synthetic, amorphous silicon 14.0 A1c AnticorrosionC303 dioxide - exchanged with calcium pigment ions Kronos ® Titaniumdioxide 3.5 White pigment 2310 for coloring Flammruβ Carbon black 0.1Black pigment 101 for coloring Talkum IT Magnesium silicate hydrate 2.0Filler extra Irgacure ® α-Hydroxy ketone 2.0 Photoinitiator 184Irgacure ® Acylphosphine oxide 0.4 Photoinitiator 819

The formulation has an acid number of about 30 mg KOH/g.

Example 2 Adhesion Primer A1 (1^(st) Layer Directly on Substrate)

Proportion in Trade the formulation Function in the name Chemicaldescription [%] Component formulation Bisphenol A diglycidyl ether 28.0A1a Binder for film diacrylate (55%), diluted with forming a) Versaticacid glycidyl ester acrylate (15%) b) dipropylene glycol diacrylate(30%) Trimethylolpropane formal 39.0 A1b Low-shrink monoacrylatereactive diluent Preparation based on: 10.0 A1b Acidic adhesion2-(phosphonooxy)ethyl promoter methacrylate,bis(methacryloyloxyethyl)hydrogenphosphate Halox ® 1.0 A1c Corrosion 650inhibitor Shieldex ® Synthetic, amorphous silicon dioxide - 14.0 A1cAnticorrosion C303 exchanged with calcium ions pigment Kronos ® Titaniumdioxide 3.5 White pigment 2310 for coloring Flammruβ Carbon black 0.1Black pigment 101 for coloring Talkum IT Magnesium silicate hydrate 2.0Filler extra Irgacure ® α-Hydroxy ketone 2.0 Photoinitiator 184Irgacure ® Acylphosphine oxide 0.4 Photoinitiator 819

The formulation has an acid number of about 30 mg KOH/g.

Example 3 Primer A2 (2^(nd) Layer)

Proportion in Trade the formulation Function in the name Chemicaldescription [%] Component formulation Difunctional, elastic, 19.0 A2aBinder for film forming aliphatic urethane acrylate, having a molecularweight of about 1500 g/mol, diluted in trimethylolpropane formalmonoacrylate Trifunctional aliphatic 38.0 A2a Binder for film formingurethane acrylate having a molecular weight of about 800 g/mol, dilutedin HDDA Laromer ® Radiation-curing, 10.0 A2a Dual-cure resin for LR 9000polymeric, acrylate- improving interlayer modified isocyanate adhesionLaromer ® Hexanediol diacrylate 11.0 A2b Difunctional reactive HDDAdiluent Laromer ® Trimethylolpropane 10.0 A2b Trifunctional reactiveTMPTA triacrylate diluent Byk ® 361 Acrylate copolymer 0.2 Flow controladditive Byk ® 020 Polysiloxane solution 0.2 Defoamer Kronos ® Titaniumdioxide 2.1 White pigment for 2310 coloring Flammruβ Carbon black 0.1Black pigment for 101 coloring Talkum IT Magnesium silicate 6.0 Fillerextra hydrate Tinuvin ® N-Alkyl-HALS 1.0 Light stabilizer 292 Irgacure ®α-Hydroxy ketone 1.0 Photoinitiator 184 Irgacure ® Acylphosphine oxide1.4 Photoinitiator 819

Example 4 Clear Coat C: (4^(th) Layer)

Proportion in the formulation Function in the Trade name Chemicaldescription [%] Component formulation Difunctional elastic, 28.7 C1Binder for film aliphatic urethane forming acrylate having a molecularweight of about 1500, diluted in trimethylolpropane formal monoacrylateTrifunctional aliphatic 44.4 C1 Binder for film urethane acrylateforming having a molecular weight of about 800 g/mol, diluted in HDDAHexanediol diacrylate 21.3 C2 Difunctional reactive diluent Byk ® 361Acrylate copolymer 0.2 C3 Flow control additive Tinuvin ® 400Hydroxyphenyltriazine 1.5 C3 Light stabilizer Tinuvin ® 292 N-Alkyl-HALS0.9 C3 Light stabilizer Irgacure ® 184 α-Hydroxy ketone 2.5Photoinitiator Lucirin ® TPO Acylphosphine oxide 0.5 Photoinitiator

Formulations (A1) and (A2) were applied to cast aluminum test castingsmeasuring about 8×12 cm by pneumatic spraying using a hot spray gun atabout 70° C. in a wet film thickness of about 50-70 μm.

After about 1 minute, formulation (A1) was irradiated and cured at abelt speed of 10 m/min and at a distance of about 10 cm with amedium-pressure mercury lamp (160 W/cm) and also with a Ga-dopedmedium-pressure mercury lamp (160 W/cm) under a nitrogen atmosphere(residual oxygen content <5000 ppm). Formulation (A2) was then alsoirradiated and cured in the same way.

Formulation (B) was applied by pneumatic spraying at room temperatureand dried physically at 60° C. for 10 minutes.

Like formulations (A1) and (A2), formulation (C) was appliedpneumatically using a hot spray gun and, after about 1 minute, wasirradiated and cured at a belt speed of 5 m/min and at a distance ofabout 10 cm with a medium-pressure mercury lamp (160 W/cm) and also witha Ga-doped medium-pressure mercury lamp (160 W/cm) under a nitrogenatmosphere (residual oxygen content <5000 ppm).

After storage for about 7 days at 21° C. and 50% relative humidity, thecoatings tests were conducted, and gave the following results:

CASS test:DIN EN 4628-8 subfilm corrosion—0.9 mmDIN EN 4628-3 surface rust—Ri0DIN EN 4628-2 blistering/degree of blisters—0 S0

Cross-cut DIN EN ISO 2409—Gt1

Multistone impact DIN EN ISO 12567-1, method B—characteristic value 1.0

1. A method for coating light alloy rims, which comprises coating amachined light alloy rim blank comprising a cast aluminum alloy iscoated with at least three layers, comprising primer (A), base coat (B),and clear coat (C), each of which are different coating materials, theprimer (A) comprising at least one radiation-curable coating materialhaving an acid number in accordance with DIN EN ISO 3682 (bypotentiometry) of 10 to 120 mg KOH/g, said coating material beingapplied to the substrate directly, and a radiation-curable clear coatlayer having a double-bond density of free-radically polymerizablereactive groups per unit mass of coating material of at least 1 mol/kgbeing applied to the outside of the light alloy rims, and curing saidcoating material by radiation.
 2. The method according to claim 1,wherein the individual layers are applied, then only dried andoptionally subjected to not more than partial curing, and full curing isnot carried out until the end.
 3. The method of claim 1, wherein theprimer (A) comprises at least one radiation-curable coating materialcomprising (A2a) at least one binder selected from the group consistingof aliphatic urethane (meth)acrylates, aromatic urethane(meth)acrylates, and epoxy (meth)acrylates, (A2b) at least one reactivediluent, and (A2c) preferably at least one anticorrosion pigment and/orcorrosion inhibitor.
 4. The method of claim 1, wherein the layer for thebasecoat (B) comprises at least one water-based, two-component coatingmaterial comprising at least one pigment and/or metallic flakes.
 5. Themethod of claim 1, wherein the layer for the basecoat (B) comprises atleast one water-based, radiation-curable coating material comprising atleast one pigment and/or metallic flakes.
 6. The method of claim 4,wherein the coating material for the layer for the basecoat (B) has asolids content of between 20% and 80% by weight.
 7. The method of claim1, wherein the curing of the radiation-curable coating material iscarried out under inert gas.
 8. The method of claim 1, wherein, betweencuring of the basecoat (B) and application of the clearcoat (C), thecured basecoat (B) is removed on the outer facing side of the rim downto the bare metal, and clearcoat is applied to this metal layer.
 9. Themethod of claim 5, wherein the coating material for the layer for thebasecoat (B) has a solids content of between 20% and 80% by weight.